The SVS guy is probably right. The Velodyne was likely all harmonic distortion. It might have seemed louder, but it was all noise garbage, and not true to the signal. The SVS will have a far higher fidelity to the original sound, and will be much more powerful besides. A couple things I would recommend to get stronger bass- try different spots for the subwoofer, if you can. Placement makes a big difference in the sound of the subwoofer with respect to the listening position. Put the sub in as many different spots as you can and play back a sweeping test tone like this one. It helps to have a SPL meter to record the values here. If you really want accuracy, use a SPL meter in conjunction with REW.

Also, if you want string bass, just run the sub hot. You might hike it up by 6 dB in the receiver's LFE output. In addition to that, run the sub with all ports open, that will give you the most headroom. Make sure the sub's amp settings are geared for all ports open if you do that. To get more out of your sub, you can also raise the crossover point. Most systems automatically crossover at 80 Hz, but you can go a lot higher. It does make the sub more localizable, but the sub will definitely be louder. Try 100 Hz, 120, 150. This is not commonly advised, but you may prefer the sound of a higher crossover.

Can you try a Y on the back of your SVS or hook it up to the SMS. Yes I have seen Yam’s having problems with the SW-out over the years. Nothing recently though that comes to mind.
EDIT> It is hooked up to your SMS. Try hooking up the SVS directly to your yam and be sure your SW trim is around -4 to start out with.

if you place a functional sub next to a non-functional sub the sub that's not working will act as an acoustic 'sponge', soaking up some of the output of the working sub. The only way to fairly test subs in-room is one at a time, after performing room response correction.

The SVS PB12-Plus is a beast so this definitely sounds like a placement and/or setup issue to me. Not all subs work best in the same spot in any given room so a sub crawl is in order. Also, ensure the levels are matched as the gain varies from sub to sub.